RPE typically incorporates one-way valves in the inhalation and/or exhalation flowpaths of the equipment to regulate the gas flow to and from the user so that clean respirable gas is inspired and waste gas is expired, without the risk of contamination from the environment or significant rebreathing of exhalate. A common form of one-way valve for this purpose is the so-called mushroom or flapper valve which in one form comprises a circular elastomeric disc fixed to a seat at its centre and is adapted to operate in one of two ways when subject to a pressure differential in the opening direction. That is to say it can either “butterfly” with opposite “wings” of the disc lifting off the seat at its periphery at low flow rates and then at higher flow rates the whole of the disc lifting, or the whole of the disc lifts off the seat around the whole periphery at all flow rates, the amount of lift being a function of the rate of flow. The second case is the more beneficial as it gives even flow around the disc. In practice the disc can be in the form of a smooth dome (“mushroom”) or stepped circular “pyramid”, the steps helping to regulate the lifting off the seat. In any case the presence of the seat restricts the maximum flow area available through the valve, however, which negatively influences the breathing resistance of the valve, particularly at high flow rates. To compensate for this shortcoming the diameter of such valves must be relatively large, typically 1 inch (25.4 mm). This can be a disadvantage; for example when such valves are installed in the usual positions on a face piece of RPE they can reduce the space available for other elements such as a transparency or communication devices. Furthermore the ability of such valves to prevent reverse flow critically depends on the integrity of the contact of the disc with a rigid knife-edge surface around the periphery of the seat and this can sometimes be a cause of leakage particularly if dust or other particulates are present on the seat or disc.